1. Field of Invention
This invention relates to an imaging system and in particular it relates to apparatus for improving the resolution and reducing the optical alias of imaging systems which include a solid state image sensor such as a CCD sensor.
2. Description of the Prior Art
In a conventional scanned imaging system using a solid state sensor, electromagnetic radiation, generally either optical or infra red is admitted by a lens system to an array of imaging elements which may be charge coupled devices (CCD's). The CCD array is scanned and an output is taken from each element in turn which is dependant upon the light incident upon that element. This output is processed and used to modulate the display on a monitor and thus to produce an image upon the monitor. The resolution of such a system is dependant upon the size and number of the CCD elements within the sensing array. Current technology limits this size to a sensor of a maximum of perhaps 600.times.600 elements each of around 10 micron dimensions.
An image is generally viewed over two scanning fields. In the first field the 1st, 3rd, 5th etc. lines of the array are scanned and during the second field the 2nd, 4th etc. lines are scanned. The processing system then interlaces the two fields upon a display monitor and the human eye and brain performs an integrating function to see the entire image. Typically the scanning time for one field is 50 Hz and thus the whole picture is refreshed with a frequency of 25 Hz.
In order to improve the resolution of such a system, it has been suggested to shift the image position with respect to the sensor by half a pixel or integer and a half pixel during each alternate field. If the shift is made in the horizontal direction then the horizontal resolution of such a system can be effectively doubled since the slight shifting of the image tends to effectively half the length of an imaging element and thus to double the total number of elements which are apparently used to view each horizontal line of the image. In order that the picture does not flicker on the monitor it is important that, after sensing, the video signals are shifted back by half pixel or integer and one and a half times in synchronism with the shift of the image. It has been suggested to shift the image either by altering the position of the CCD slightly or to alter the angle of a glass plate or mirror placed between the lens and imaging sensor. The present invention arose from a need to maintain the resolution increase whilst also reducing the degrading effects of aliassing. Also, it was required to provide a system which can use a standard CCD camera.